Blender model is not correct in unity
In Unity to save processing power the model does not render both sides of a face, the problem is you have accidentally set the face normals the wrong way, thankfully it's easy to fix.
In Blender, select the object you need to fix and open edit mode, press a to select all and then press Option+N for Mac or Alt+N for PC. Then just select either flip normals or Recalculate outside. If you want to check if the normals are the right way without putting it into Unity you can set Backface culling to true in the materials settings window.
For example: